It is usual in the orthodontic art to use a small annular elastic ligature, or tie wire, to hold the arch wire of an orthodontic appliance in place on each of the brackets which are mounted on the teeth of the patient. These ligatures are usually applied by hand or by the use of forceps or similar instruments to stretch the elastic ligatures and to insert the ligatures into annular support grooves in the brackets.
U.S. Pat. No. 4,277,236 which issued to the present inventor discloses an appropriate hand instrument by which such elastic ligatures may be applied simply and expeditiously in a stretched condition into the annular grooves in the orthodontic brackets and around the arch wire, so that the arch wire may be supported on the brackets.
In accordance with the concepts of the present invention, appropriate thermoplastic material is used to constitute the annular ligatures. The ligatures of the invention may be applied to the orthodontic brackets by hand, or by an instrument such as disclosed in said patent. The ligatures of the invention, as mentioned above, are heated by any appropriate means after they have been applied to contract and firmly and rigidly hold the arch wire on the brackets.
An appropriate material for the annular ligature of the invention is the same thermoplastic material which is used for shrink film packaging. In the practice of the invention, a ligature formed of the material is attached to an orthodontic appliance on a tooth of a patient, and it is then heated. This causes the ligature to shrink and firmly and securely hold the arch wire in place on the orthodontic bracket.
Almost any thermoplastic material, such as, polycarbonate, polyethylene, polypropylene, or the like, are suitable to constitute the ligature. In accordance with the usual shrink film techniques, the thermoplastic material is first heated to its softened state, stretched, and then allowed to cool while in the stretched state. Subsequent heating of a material while in the mouth of the patient, to a temperature not hot enough to burn the patient, causes the inherent memory of the material to cause the ligature to shrink to its original state and to remain in its shrunken state upon subsequent cooling, so that the ligature firmly and securely retains the arch wire in the orthodontic bracket, with a high strength and force as compared with the usual prior art elastic ligatures.
Suitable materials are glass-reinforced thermoplastics such as described, for example, in an article appearing in the Feb. 4, 1971--Sept. 3, 1971, issue of Machine Design entitled "Glass Reinforced Thermoplastic". The materials are presently being sold by LNP Corporation, of Malvern, Pennsylvania.